References to Group III, IV, and V elements follow art established designations of elements found in groups 13, 14, and 15, respectively, of the Periodic Table of elements as adopted by the American Chemical Society.
Following the discovery of the transistor, semiconductor application interest focused on group IV elements, first primarily on germanium and then on silicon. It was later recognized that useful and, for many applications, superior semiconductor properties are provided by III-V compounds--that is, compounds consisting of group III and group V elements. This has led to intensive investigations of processes for preparing layers of III-V compounds, particularly processes offering the stringent control of III-V compound layer stoichiometry, purity, uniformity, and thickness required for successful semiconductor applications.
The most commonly employed approach for preparing III-V compound layers is chemical vapor deposition (CVD), which includes both vapor phase epitaxy (VPE) and metalorganic chemical vapor deposition (MOCVD). A gaseous compound of a group III element and a gaseous compound of a group V element are introduced into a vacuum chamber and thermally decomposed in the presence of a substrate. Although extensively used, this process exhibits a number of disadvantages. First, there is the safety hazard of working with toxic gases. Second, each of the group III element and group V element compounds are pyrophoric, reacting spontaneously with oxygen. Third, with the group III and group V elements being introduced as separate gases, the potential for layers which are stoichiometrically unbalanced in either the concentration of the group III or group V element is always present, and precise gas metering is required for balanced stoichiometry. Fourth, working with high vacuum equipment is time consuming, cumbersome, and operationally limiting.
Constant et al U.S. Pat. No. 4,250,205 discloses a variation on the CVD process described above. Instead of employing a gaseous compound of a group III element and a gaseous compound of a group V element as separate precursors for III-V compound deposition, a single gaseous precursor is employed which is a coordinatin compound of one group III element substituted with three volatilizable ligands and one group V element substituted with three volatilizable ligands. Such coordination compounds are also referred to in the art as III-V donor acceptor complexes and as III-V Lewis acid and Lewis base adducts. Constant et al teaches avoiding ligand elimination leading to polymeric compounds. Although the coordination compound approach offers better replication of ratios of III and V elements and to some extent ameliorates problems of toxicity and oxygen sensitivity, the limitations of using high vacuum equipment for coating remain unabated.
Zaouk et al, "Various Chemical Mechanisms for the Crystal Growth of III-V Semiconductors Using Coordination Compounds as Starting Material in the MOCVD Process", Journal of Crystal Growth, Vol. 55, 1981, pp. 135-144 discloses a variation on the process of Constant et al wherein elimination of one ligand from each of the III and V elements of the precursor is recognized to occur during heating. Maury et al, "Raman Spectroscopy Characterization of Polycrystalline GaP Thin Films Grown by MO-CVD Process Using [Et.sub.2 Ga-PEt.sub.2 ].sub.3 As Only Source", Journal de Physique, Colloque C1, suppl. no. 10, vol. 43, Oct. 1982, pp. C1-347 to C1-252, is essentially cumulative with Constant et al and Zaouk et al, except for employing polymeric precursors as starting materials. Zaouk et al and Maury et al share the disadvantages of Constant et al.
Davey U.S. Pat. No. 4,427,714 describes forming III-V compound layers by spraying. For example, gallium arsenide layers are disclosed to be formed by processes including
(1) spraying a solution of gallium arsenide or a precursor thereof with an inert gas propellant in a reducing gaseous atmosphere;
(2) spraying a solution of gallium/arsenic complex (each of the gallium and arsenic atoms having three substituent legands) with an inert gas propellant in an inert or reducing atmosphere;
(3) creating a stable aerosol of trimethyl gallium dispersed in arsine, which is sprayed on a hot substrate; and
(4) spraying a polymeric complex formed between trimethyl gallium and methyl/phenyl arsine. All of the spraying processes are unattractive, since considerable unwanted deposition occurs on spray confining walls. Thus precursor waste and burdensome cleaning of equipment is encountered.
It has been recognized that III-V compound layers can be produced by supplying liquids to substrate surfaces. Ladany et al U.S. Pat. No. 3,802,967 discloses first forming a thin III-V compound layer by CVD techniques and then increasing the thickness of this layer by conventional liquid phase epitaxy. For instance, in Example 1 a liquid consisting of 97 percent gallium, 2.99 percent gallium arsenide, and 0.01 percent tellurium is flowed over a 10 micrometer CVD GaAs layer on a spinel substrate by tipping a graphite boat containing the liquid and substrate. The temperature of the liquid is maintained at 700.degree. C. The Ladany et al process, since it begins with CVD, incurs all of the disadvantages of that process and in addition is unattractive in requiring very high temperatures for liquid phase epitaxy.
Jensen U.S. Pat. No. 4,594,264 discloses a process for preparing gallium arsenide layers on monocrystalline, gallium arsenide or silicon substrates. A gallium-arsenic complex is employed of the formula EQU (I) X.sub.3 GaAsR.sub.3
where
X is chlorine, bromine, iodine, phenyl, benzyl, methyl, or trifluoromethyl, and
R is hydrogen, phenyl, benzyl, methyl, or trifluoromethyl.
The complex is dissolved in a hydrocarbon or chlorinated hydrocarbon solvent which is free of oxygen, sulfur, and nitrogen. The resulting solution is coated as a film on the substrate in an amount sufficient to form a gallium arsenide layer of from 1 to a few micrometers (.mu.m) in thickness. The film is then heated to a temperature of less than 200.degree. C. to volatilize the solvent while avoiding decomposition of the gallium-arsenic complex. The next step of the process is to convert the complex coating remaining to gallium arsenide by exposing the coating to ultraviolet (UV) radiation, such as the UV radiation from a laser. The presence of moisture and oxygen is avoided. All reactions were carried out under an inert, dry atmosphere (typically less than 1 ppm oxygen content) using purified, dry, oxygen-free solvents. Analysis of a layer produced from a complex of C1.sub.3 GaAs(C.sub.6 H.sub.5).sub.3 revealed that it had lost only 70 percent of the carbon and 54 percent of the chlorine of its parent coating as measured prior to UV exposure. These residual carbon and chlorine levels are, of course, unacceptably high for the majority of semiconductor uses.